Separation of waxy hydrocarbons from oils by flotation

ABSTRACT

In the separation of waxes from oils by flotation at a temperature below the cloud point the oil is cooled by withdrawing a stream, cooling it and recycling it in a ratio to fresh oil of at least 5:1. Other details are as described in Specification 1,022,429.

1%? P. A. TRITSMANS SEPARATION OF WAXY HYDRQCARBONS FROM OILS BY FLOTATION Filed Feb. 16, 1965 DEM/4X50 OIL 8 COOL/4N T INVENTOR. PAUL ALBERT TRITSMANS BY MORGAN, FINNEGAN, DURHAM 8s PINE EED I ATTORNEYS United States Patent 3 Claims. b1. 208-28) This invention relates to the separation of waxy hydrocarbons from oils by flotation.

It is known to separate waxy hydrocarbons from oils by cooling in the presence of solvents and separating the solid wax by filtering. It has also been proposed to separate waxy hydrocarbons by flotation. In this process bubbles of gas are passed upwardly through the cooled oil so that the Wax separates as a froth at the top of the oil. This process has the advantage that no filtering or centrifuging is required and it is in essence, a simple process. Nevertheless, as far as is known, the process has never been developed on a large scale. One reason for this lack of development may be due to the critical physical conditions which are needed for satisfactory results and the fact that different feedstocks vary markedly in the ease with which they can be dewaxed by flotation.

This aspect is discussed in co-pending US. patent application Ser. No. 433,180, filed Feb. 16, 1965, and which corresponds to Belgian application No. 43,900.

Another and equally important reason lies in the practical difficulties of cooling the oil on a large continuous scale. In a flotation process the gas bubbles must have an unimpeded passage through the oil so that cooling coils in the flotation bath cannot be used. Equally, cooling the oil before it reaches the bath is undesirable since wax deposition is likely to occur and scrapers may have to be employed.

One previous proposal which avoids these difficulties suggests that the gas used should be pre-cooled and used also to cool the oil, but a consideration of the specific heats of suitable gases and oils likely to require treatment shows that a very high degree of cooling is required at any reasonable gasroil ratios.

The present invention, therefore, provides a simple alternative method of cooling the oil in a wax flotation process.

According to the present invention a process for the separation of waxy hydrocarbons from oil by flotation in which gas is passed upwardly through the oil cooled to a temperature below its cloud point is characterised in that fresh oil is fed to the flotation zone at a temperature above its cloud point, and that the fresh oil is cooled by withdrawing a stream of oil from the Zone, cooling this stream and recycling it to the zone, the ratio of recycled oil to fresh oil being at least 5:1, preferably :1.

The use of recycle cooling at a high recycle: fresh oil ratio means that the extent of cooling necessary for the recycle oil is low, being in practice of the order of 6 F. or less, and also that the rate of flow through the coolers can be high. Both these factors will tend to minimise deposition of wax in the coolers and the need for scraped coolers is avoided. However, if a safeguard is considered necessary to allow for start up or malfunctioning, coolers can be arranged in parallel, with means for feeding the oil to all or any of the coolers. In this way one or more coolers can be taken off stream for cleaning without shutting down the whole plant. Cleaning can be carried out simply and easily by warming the cooler with for example warm oil.

The coolers for the recycle oil can be of any convenient type. One example of suitable coolers is tubular heat ex- 3,347,775 Patented Oct. 17, 1967 changers. Similarly the choice of coolant is not critical; liquefied petroleum gases such as liquid propane or butane are convenient coolants.

Preferably the stream of recycle oil is withdrawn from the flotation zone at a point removed from the fresh oil inlet and preferably also it is withdrawn from the bottom of the zone. The recycle oil may bereturned to the flotation zone via a separate line entering the bath preferably at a point close to the fresh oil inlet. However it has been found that a single recycle and fresh oil line may be used provided this line is not too long, the recycle and fresh oil being then conveniently mixed in or near the'fresh oil and recycle pumps.

Dewaxed oil product must clearly be withdrawn in an amount suflicient to keep a constant liquid in the zone and this may conveniently be achieved by bleeding off the required amount from the recycle stream. Wax, which collects at the top of the zone is also withdrawn continuously or at intervals. A particularly convenient way of removing the wax froth is by means of an arm extending across the length or breadth of the bath and which traverses along the top of the bath. This may be caused to traverse for example by attaching it to an endless belt. The arm pushes the wax over a weir into a collecting chamber, which may conveniently be thermally insulated from the zone and contain a heating coil. The warmed wax can then be readily withdrawn as a liquid.

In co-pending U.S. patent application Ser. No. 433,180, filed Feb. 16, 1965, and which corresponds to Belgian application No. 43,900 describes the importance of the correct choice of feedstock and the feedstocks are thus desirably those containing at least a proportion of cracked material, more particularly 0.5 to 100% vol. of cracked material, and having a viscosity of not more than 20 centistokes at the separation temperature described in that application. It has been found that the recycle rate should be as high as possible in order to reduce the likelihood of wax deposition from the recycle stream.

Waxy hydrocarbons can cause in the kerosine, gas oil and wax to 650 C.)

also been found that the presence of asphalt or other materials functioning as pour point depressants is deleterious, even in small quantities, and their presence should thus be avoided. The cloud points of the oils being treated may vary over a fairly wide range and hence the flotation temperature may also vary. Likely temperatures will however be in the range 0 F. to F., especially 20 F. to 40 F. The cloud point of the dewaxed oil will with efiicient operation be substantially equal to the temperature used.

The amount of gas used is not results have been obtained with a to oil feedstock of as low as 0.2 and with no indication that this is a limiting ratio. Higher rates are not detrimental although they add little further benefit and a suitable range is thus from 0.1 to 10. Any convenient gas may be used, for example nitrogen, gases rich in nitrogen such as flue gas, hydrogen or normally gaseous hydrocarbons. Air is also suitable, but its use may be limited on a commercial scale by safety regulations regarding fire hazard.

The gas should naturally pass upwardly through the oil in the form of bubbles and if necessary it can be introduced into the bottom of the flotation bath through known bubblers and dispersers. It has been found, however, that the gas can be introduced quite simply and effectively by injecting it into the suction side of the pump feeding oil to the bath, taking care to obtain a pump discharge pressure between 2 and 10 kg./cm.

critical and satisfactory ratio by volume of gas The rate of flow of the oil into and the withdrawal of dewaxed oil from the flotation zone should be adjusted to give a suitable residence time and these can readily be determined by experiment. In practice residence times of upwards of 7 hours have been found suitable.

From the present process one can obtain a dewaxed oil of reduced cloud and pour points and also wax. When in the form of froth at the top of the flotation zone the wax may contain significant quantities of entrained oil. However, much of this oil separates on standing and any oil still remaining can be further separated by known techniques to give waxes equivalent in quality to Waxes separated for example by filtration in the presence of solvents. If the wax is not required for specialised purposes it may be used, for example, as a high quality catalytic cracker feedstock.

The invention is illustrated With references to the accompanying drawing which is a flow diagram of a suitable plant.

In the drawing, the flotation zone is a rectangular tank 1 with an oil inlet 2 and outlet 3. Fresh feed is fed into the system at 4 together with gas which is injected through line 5 into the suction side of pump 6. The fresh feed is mixed with a recycle stream described in more detail below and passes via line 7 to the inlet 2.

A stream of oil is withdrawn through outlet 3 and line 8. A portion of this stream is bled off through line 9 as dewaxed oil product and the remainder is passed through a series of heat exchangers 10, 11, 12. By means of suitable pipeworks and valves indicated diagrarnatically in the figure the oil can be fed to all or any of the exchangers. The oil cooled in the exchangers goes via line 13 to join the fresh feed and flotation gas and hence back to the tank 1 via inlet 2. Pumps are placed in this recycle line as appropriate for example at 25 and 26.

The heat exchangers are supplied with coolant via line 14 which again may go to all or any of the exchangers and which is withdrawn via line 15.

On the oil side of the exchangers pipe and valve work may be added to allow a cleaning fluid such as warm oil to pass through any exchanger which has been isolated from the system.

The level of oil in the tank is maintained constant by balancing the supply and withdrawal of oil at a level just below that of the weir 16. An endess belt 17 driven by pulleys 18 and 19 has an arm 20 extending across the breadth of the bath. At the end of the bath having the weir is a wax collecting chamber 21 with a heating coil 22 and outlet 23. The side of the bath against the collecting chamber has thermal insulation 24.

Example In a specific operation, tank 1, which was 25 metres long, 12 metres broad and 1.5 metres deep was filled with oil at 70 F. The oil was a catalytic cracker cycle oil of 320 to 370 C. boiling range and 50 centistokes viscosity at 32 F. diluted with straight run kerosine to give an oil of 13 centistokes viscosity at 32 F., 62 F. cloud point, 60 F. pour point and 170 to 370 C. boiling range. The oil was then cooled to 30 F. by recycling it via line 8 through heat exchangers 10, 11, 12 and back to the tank via lines 13 and 7. At this initial cooling stage wax deposition in the exchangers may occur and exchangers may be isolated from the system as necessary and cleaned with a separate stream of warm oil.

Alternatively, initial start-up may be carried out by filling the settling vessel with kerosine, cooling the kerosine to the required temperature and only then introducing the feed oil. Liquid propane at 20 F. is passed .41 through the other side of the exchangers via lines 14 and 15.

When the oil had reached 30 F. and gas was introduced via lines 4, 5 and pump 6. The fresh oil was cooled to the tank temperature of 32 F. by the recycle stream. The gas injected at 5 at the pressure of the suction side of pump 6 was liberated when the oil reached tank 1 and passed upwardly in the form of fine bubbles. At the same time wax separated from the fresh oil and coalesced with the bubbles to form a froth on the oil surface. This was pushed at regular intervals by arm 20 over weir 16 into the collecting chamber 21, where heating coil 22 liquefied it and allowed it to be withdrawn through line 23.

The fresh oil feed rate was 10 m. /h., the ratio of recycle oil to fresh oil was 20:1 and the temperature drop of the recycle oil through the exchangers was 2 F. The gas, which was flue gas, was used in a ratio to fresh oil of 0.5: 1.

Yields of dewaxed oil having a cloud point of 30 F. were and the yield of wax froth was 20%.

I claim:

1. A continuous process for the flotation separation of waxy hydrocarbons from waxy oil to produce dewaxed oil having reduced cloud and pour points which comprises passes fresh waxy oil into a flotation zone at a temperature above its cloud point, withdrawing a portion of oil from said flotation zone at a point remote from the fresh oil inlet of the flotation zone, passing the withdrawn oil through heat exchanger means at a rate of flow to minimize any wax deposition thereon and to cool the withdrawn oil to a temperature at which when recycled for admixture with fresh oil, the resulting mixture will be at a temperature at which wax separation will occur, recycling the cooled oil back to the flotation zone at a point to the fresh oil inlet to cool the oil present therein to a temperature below the cloud point of the oil, the ratio of recycled cool oil to fresh oil into said flotation zone being at least 5:1, passing a flotation gas upwardly through the cooled waxy oil in said flotation zone to separate the mixture into a top waxy froth layer and a bottom dewaxed oil layer, and withdrawing the dewaxed oil from the separation system by bleeding same off said recycle stream in an amount to keep a constant quantity of oil in said flotation zone.

2. A process as claimed in claim 1 wherein the wax froth is removed by pushing it over a weir at one side of the zone into a collecting chamber, heating the froth in the chamber and withdrawing it as a liquid.

3. A process in accordance with claim 1 wherein the ratio of recycled cool oil to fresh oil into the said flotation zone is 10:1.

F., fresh oil feed at 70 References Cited UNITED STATES PATENTS 1,994,485 3/1935 Perkins et a1. 208-37 2,314,994 3/1943 Lawson et al. 208-33 2,370,453 2/1945 Dons et a1. 208-33 2,410,483 11/1946 Dons et a1. 208-33 2,719,817 10/1955 Doorn 208-37 2,813,074 11/1957 Banks et al. 210-221 2,880,159 3/1959 Livingstone et a1. 208-37 2,904,496 9/1959 Bloch et a1 208-28 2,961,397 11/1960 Moscrip 208-37 FOREIGN PATENTS 734,123 7/ 1955 Great Britain.

DANIEL E. WYMAN, Primary Examiner. P. KONOPKA, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,347,775 October 17, 1967 Paul Albert Tritsmans It is hereby certified that error appears in the above numbered pat ent requiring correction and that the said Letters Patent should read as corrected below Column 4, line 26, for "passes" read passing line 36, for "point to" read point close to Signed and sealed this 14th day of January 1969.

(SEAL) Attest:

EDWARD J BRENNER Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

1. A CONTINUOUS PROCESS FOR THE FLOTATION SEPARATION OF WAXY HYDROCARBONS FROM WAXY OIL TO PRODUCE DEWAXED OIL HAVING REDUCED CLOUD AND POUR PINTS WHICH COMPRISES PASSES FRESH WAXY OIL INTO A FLOTATION ZONE AT A TEMPERATURE ABOVE ITS CLOUD POINT, WITHDRAWING A PORTION OF OIL FROM SAID FLOTATION ZONE AT A POINT REMOTE FROM THE FRESH OIL INLET OF THE FLOTATION ZONE, PASSING THE WITHDRAWN OIL THROUGH HEAT EXCHANGER MEANS AT A RATE OF FLOW TO MINIMIZE ANY WAX DEPOSITION THEREON AND TO COOL THE WITHDRAWN OIL TO A TEMPERATURE AT WHICH WHEN RECYCLED FOR ADMIXTURE WITH FRESH OIL, THE RESULTING MIXTURE WILL BE AT A TEMPERATURE AT WHICH WAX SEPARATION WILL OCCUR RECYCLING THE COOLED OIL BACK TO THE FLOTATION ZONE AT A POINT TO THE FRESH OIL TO COOL THE OIL PRESENT THEREIN TO A TEMPERATURE BELOW THE CLOUD POINT OF THE OIL, THE RATIO OF RECYCLED COOL OIL TO FRESH OIL INTO SAID FLOTATION ZONE BEING AT LEAST 5:1, PASSING A FLOTATION GAS UPWARDLY THROUGH THE COOLED WAXY OIL IN SAID FLOTATION ZONE TO SEPARATE THE MIXTURE INTO A TOP WAXY FROTH LAYER AND A BOTTOM DEWAXED OIL LAYER, AND WITHDRAWING THE DEWAXED OIL FROM THE SEPARATION SYSTEM BY BLEEDING SAME OFF SAID RECYCLE STREAM IN AN AMOUNT TO KEEP A CONSTANT QUANTITY OF OIL IN SAID FLOTATION ZONE. 